The energy density of a storage device refers to the amount of energy that can be stored in a given volume or weight of the device. The power density refers to the manner in which energy can be stored in the device. The greater the power density, the faster the storage device can be charged and discharged. In the field of energy storage, the energy density and power density are both of critical importance.
An ideal storage device will possess both a high energy density and a high power density. Unfortunately, most storage devices achieve only one of these characteristics. In particular, with respect to batteries, the energy density is high but the power density is characteristically low. The power density of a battery is controlled by the rate at which chemical reactions occur and there is little ability to change that. On the other hand, capacitors have a limited energy density but a high power density.
Much of the research in the field of energy storage has been directed towards developing storage devices that exhibit both high energy and power density. For example, research into conducting polymers for battery applications has lead to improvements in power density for batteries. Developments include a nano-structured high-surface area electrode material composed of cellulose fibers that are individually coated with a very thin layer of polypyrrole.
A nanocomposite of LiFePO4 nanoparticles embedded in a nano-porous carbon matrix as a cathode material reportedly provides improved power density for lithium-ion batteries. Lithium-ion batteries absorb and release energy via the removal and insertion of Li+ ions and electrons. The power density of a lithium battery depends on the rate at which the ions and electrons can move through the electrolyte and electrode structure into the active electrode material. The LiFePO4 nanoparticles embedded in a nano-porous carbon matrix improve power by improving electron transport in the bulk or at the surface of the material, or on reducing the path length over which the electron and the Li+ ion have to move
Supercapacitors are electrochemical, double-layer capacitors. Supercapacitors include two electrodes, a separator, and an electrolyte. Energy is stored by charge transfer at the boundary between the electrode and electrolyte. The amount of stored energy is a function of the available electrode surface, the size of the ions, and the level of the electrolyte decomposition voltage. Although an electrolyte is present, the principle of operation for supercapacitors is based on electrostatics, not chemical reactions. As a consequence, the power density is higher than batteries. Yet, supercapacitors generally have a lower energy density than batteries.
Notwithstanding these improvements, a need remains for an energy storage device that provides high energy and power density.